1. Field of the Invention
The invention relates generally to producing thin films of tin and/or fluorine doped indium oxide and especially to doing so with atmospheric pressure chemical vapor deposition (APCVD) from the indium chemical sources of trimethylindium or trimethylindiumdiethyletherate, the tin chemical source tetramethyltin, the fluorine chemical source bromotrifluoromethane, and the oxygen sources of molecular oxygen (O.sub.2) or water.
2. Description of the prior Art
Tin doped indium oxide (indium-tin-oxide, or ITO) films exhibit the relatively rare combination of transparency to light and good electrical conductivity. Transparent conductive oxides (TCOs), such as ITO films, are commonly used in liquid crystal, electroluminescent, plasma, and vacuum fluorescent information displays. TCOs are also useful in photocells and the gate electrodes in charge injection devices (CIDs) and charge coupled devices (CCDs) used in such places as video cameras. As transparent electrical resistors, such layers are used for defrosting windows in airplanes, cars, etc. The heat reflecting property of such films on glass substrates has been used to enhance the efficiency of solar thermal collectors, building windows, ovens, furnaces, sodium-vapor lamps, and fiberglass insulation.
Stannic oxide (SnO.sub.2), indium oxide (In.sub.2 O.sub.3), and cadmium stannate (Cd.sub.2 SnO.sub.4) films have been produced by a wide variety of techniques that include sputtering, evaporation, and spray pyrolysis. U.S. Pat. No. 4,265,974, issued May 5, 1981, to Gordon provides a background of the many ways that have been used to produce transparent, electrically conductive coatings and layers. The earliest methods of applying these coatings were based on spraying a solution of a metal salt on a hot surface, such as glass. U.S. Pat. No. 2,617,745 describes a way to mitigate the fogging of the glass by applying a coating of pure silica on the glass. A problem with the uniformity and reproducibility in such coatings was addressed by controlling the humidity in the apparatus, in U.S. Pat. No. 2,651,585. Cleaner, more reproducible coatings have been attempted with vacuum deposition techniques, such as evaporation and sputtering. Semiconductor fabrication demands are more than enough to justify the higher costs associated with such techniques.
Impurities of tin, antimony, and fluorine in semiconductors will contribute to the conductivity of a device and can be deliberately introduced during the application of the above coatings and films. Fluorine has an advantage over antimony as a dopant for tin oxide, in that the transparency and conductivity of the fluorine-doped stannic oxide films is higher than that of the antimony-doped ones. Gordon points out that fluorine doping has only been demonstrated in the less satisfactory spray method. CVD, evaporation, and sputtering are not believed to have been shown to produce fluorine doping. The advantage of fluorine is important to solar cell and solar collector applications, among others. By using a spray method, Gillery, as described in U.S. Pat. No. 3,677,814, achieves one of the lowest resistivities in tin oxide films. Using a spray method, Gillery obtained fluorine-doped tin oxide films with resistances as low as fifteen ohms per square. Gordon observed that the lowest commercially available tin-oxide coated glass was, in 1979, in the range of forty ohms per square.
Further improvements in methods and equipment are needed to reduce the expense of vacuum deposition of tin-oxide and indium-oxide films and to improve both the electrical conductivity and visible light transparency.
Oxides of metals can usually be expected to act as good electrical insulators. Tin-oxide is no exception, however oxygen deficient tin-oxide exhibits a degree of electrical conductivity and is transparent to visible light when deposited as a film. Indium-oxide is another material that exhibits a degree of electrical conductivity and is transparent to visible light when deposited as a film. In a binary system the indium chemical, In(CH.sub.3).sub.3 O(C.sub.2 H.sub.5).sub.2, reacts with molecular oxygen, O.sub.2, in the theoretically stoichiometric way: EQU 2In(CH.sub.3).sub.3 O(C.sub.2 H.sub.5).sub.2 +24O.sub.2 .fwdarw.In.sub.2 O.sub.3 +14CO.sub.2 +19H.sub.2 O (1)
In a first tertiary system tin (Sn) substitutes for indium (In) as a cation electron donor, such that electrical conductivity can be more reliably controlled in manufacturing, as in (2), below. Two minus "x" In(CH.sub.3).sub.3 O(C.sub.2 H.sub.5).sub.2 molecules combine with "x" number of Sn(CH.sub.3).sub.4 molecules, and an appropriate number of O.sub.2 molecules to form indium-tin-oxide (ITO) with "x" number of tin atoms substituting for indium atoms. EQU (2-x)In(CH.sub.3).sub.3 O(C.sub.2 H.sub.5).sub.2 +xSn(CH.sub.3).sub.4 +(24-17x/4)O.sub.2 .fwdarw.2In.sub.2-x Sn.sub.x O.sub.3 +(14-3x)CO.sub.2 +(19-7x/2)H.sub.2 O (2)
In a second tertiary system fluorine (F) substitutes for oxygen (O) as an anion electron donor, and is again such that electrical conductivity can be more reliably controlled in manufacturing, as in (3), below. Two In(CH.sub.3).sub.3 O(C.sub.2 H.sub.5).sub.2 molecules combine with "y" divided by three number of CF.sub.3 Br molecules, and an appropriate number of O.sub.2 molecules to form indium-flourine-oxide (IFO) with "y" number of fluorine atoms substituting for oxygen atoms. EQU 2In(CH.sub.3).sub.3 O(C.sub.2 H.sub.5).sub.2 +(y/3)CF.sub.3 Br+(24-y/4)O.sub.2 .fwdarw.In.sub.2 O.sub.3-y F.sub.y +(14+y/3)CO.sub.2 +(19-y/6)H.sub.2 O+(y/3)HBr (3)
In a quaternary system that essentially combines reactions (2) and (3) above, tin (Sn) substitutes for indium (In), and fluorine (F) substitutes for oxygen (O) in a system resulting in dual electron donors. As shown in equation (4), below, two minus "x" number of In(CH.sub.3).sub.3 O(C.sub.2 H.sub.5).sub.2 molecules combine with "x" number of Sn(CH.sub.3).sub.4 molecules, and "y" divided by three number of CF.sub.3 Br molecules, and an appropriate number of O.sub.2 molecules to form indium-tin-oxide doped with fluorine (ITO:F) with "x" number of tin atoms substituting for indium atoms, and "y" number of fluorine atoms substituting for oxygen atoms. EQU (2-x)In(CH.sub.3).sub.3 O(C.sub.2 H.sub.5).sub.2 +xSn(CH.sub.3).sub.4 +(y/3)CF.sub.3 Br+(24-17x/4-y/4)O.sub.2 .fwdarw.In.sub.2-x Sn.sub.x O.sub.3-y Fy+(14-3x+y/3)CO.sub.2 +(19-7x/2-y/6)H.sub.2 O+(y/3)HBr(4)
The theoretical proportions of the various chemicals listed in equations (1) to (4) may, or may not, be realized in actual practice. The actual practical proportions for the chemicals depends upon the exact reaction conditions, and the reaction equipment used.
Such a system lends itself to continuous atmospheric pressure chemical vapor deposition techniques. The method and equipment embodiments of the present invention described below offer a practical and economical way to manufacture films on glass or other suitable substrates that are electrically conductive and transparent to visible light.